The present invention relates to a flowmeter wherein the rate of flow of fluid through a conduit is measured electrically.
It is known that a thermocouple may be connected to a heated conduit in order to measure fluid flow therethrough by sensing changes in thermoelectric voltages resulting from the flow. Such an arrangement is disclosed, for example, in U.S. Pat. No. 3,181,357 which issued to James M. Benson on May 4, 1965.
U.S. Pat. No. 3,229,522 granted to James M. Benson on Jan. 18, 1966 also teaches the use of thermocouple circuits for sensing the temperature variations in a heated conduit by measuring thermoelectric voltages.
With prior art arrangements of the type just described, the thermoelectric junctions are formed by wires of one material attached to a conduit of a different material selected to allow a useful thermoelectric e.m.f. to be developed at the junction. However, flowmeters of this type are limited to those materials which have useful thermoelectric characteristics. An additional shortcoming is that such flowmeters produce a d.c. thermoelectric output which inherently is more difficult to amplify than a.c.
Another method of determining fluid flow in a heated conduit measures the resistance of the conduit itself. An example of such an arrangement is that disclosed in U.S. Pat. No. 3,818,758 which was granted to Edmond Easter on June 25, 1974. In this patent, there is described a flowmeter wherein the change in resistance of a heated conduit resulting from fluid flow therethrough is detected by a Wheatstone bridge circuit connected to a point on the conduit. The resistance variations are a function of the flowrate. Therefore, the response of the bridge to resistance changes is utilized to measure the flow.
However, the changes in resistance of a heated conduit resulting from fluid flow therethrough are quite small and are not uniform over the length of the conduit. Consequently, a Wheatstone bridge connected to a single point along the conduit measures the total resistance change of the conduit thereby producing a measurement of the average resistance change per unit flow rather than the maximum change. Thus, the Wheatstone bridge measurement is inherently a somewhat inaccurate one.